http://www.pirate4x4.com/tech/billavista/Brakes/
You might be better off looking at Billavista's brake bible. You need to start with matching the master to the front calipers and then go from there either up or down in size. There's a crazy amount of info in it. What's the bore size of your master? Front caliper piston surface area? blah, blah, blah.
If I remember right, vette masters are at best 1" bore. 1/2 ton and 3/4 ton calipers are the exact same. Only the rotor is different. I've got this setup front and back running a 1-1/8" master with 40's and love it. This was after ditching the 1".
don't go too large though. Larger=more volume and less pressure. (Hard thigh burning pedal but not enough pressure to stop)